Stand behind a jet at full power and the experience is less like hearing a machine than being shoved by one. A modern airliner engine can produce more than 100,000 pounds of thrust from a device with essentially no reciprocating parts — just a shaft, some blades, and fire. And underneath the engineering, the idea is almost embarrassingly simple.

A jet engine grabs a great deal of air, throws it backwards very hard, and gets pushed forwards in return. Everything else is detail. Here is how that detail works.

Suck, squeeze, bang, blow

Generations of engineers have learned the gas turbine through four words.

Suck. Air enters the intake, funnelled in by the fan at the front. Squeeze. That air passes through the compressor — rank after rank of spinning blades, each stage raising the pressure, until the air is many times denser than the atmosphere outside. Bang. In the combustor, fuel is sprayed into that high-pressure air and burned continuously. Blow. The hot, expanding gas rushes rearward through the turbine and out of the nozzle.

The clever part is that the turbine sits in that exhaust stream and is spun by it — and the turbine is connected by a shaft to the compressor and fan at the front. The engine therefore powers its own air supply. Once it is running, it keeps itself running.

The thermodynamics, briefly

Formally, this is the Brayton cycle: compress a gas, add heat at roughly constant pressure, then expand it to extract work. The engine converts chemical energy in kerosene into a fast-moving column of gas. What escapes the nozzle carries far more momentum than the still air that entered the intake, and by Newton’s third law that difference is thrust.

Which points at a question with a counter-intuitive answer: is it better to throw a small amount of air very fast, or a lot of air moderately fast?

Why modern engines are mostly fan

Early jets were turbojets: every scrap of air went through the burning core and out the back at enormous velocity. Effective, thirsty, and deafening. Then engineers realised it is far more efficient to accelerate a large mass of air a little than a small mass of air a lot.

Hence the turbofan. A huge fan at the front pushes most of its air around the core rather than through it. That bypass air never burns; it simply gets shoved rearward, producing the majority of the thrust on a modern airliner engine. The ratio between bypass and core flow — the bypass ratio — runs from about 5:1 on older CFM56s to more than 12:1 on the newest GE9X. Higher bypass means better fuel burn and far less noise, which is why airliner engines have grown into the enormous barrels you see today, while fighters keep slim, low-bypass engines built for speed.

Hotter than the metal it is made of

Here is the detail that surprises people most: the gas entering the turbine is hotter than the melting point of the turbine blades. Efficiency rises with turbine temperature, so engineers push past what the metal can survive and then cheat. Blades are grown as single crystals with no grain boundaries to fail along, coated in ceramic thermal barriers, and riddled with tiny holes that bleed cool compressor air over their surfaces as a protective film. The blade never quite touches the hottest gas.

Each of those blades, incidentally, is extracting power comparable to a Formula 1 car — while glowing, spinning at tens of thousands of rpm, and holding on against centrifugal loads measured in tonnes.

The man who started it

The whole architecture traces back to two people who worked it out independently: Hans von Ohain in Germany and Frank Whittle in Britain. Whittle first ran his Power Jets WU on 12 April 1937, and the experience was not soothing.

Britain’s first jet, the Gloster E.28/39, flew on 15 May 1941. Within a decade the piston fighter was obsolete; within three, the jet had shrunk the world. Every turbofan hanging under a wing today is a direct descendant of that shrieking, red-hot machine in a workshop in 1937.

Sources: Rolls-Royce; NASA Glenn Research Center; GE Aerospace; Imperial War Museums; Popular Science.